1. Technical Field
The present invention relates to a photoresist composition and, more particularly, to a negative photoresist composition involving non-crosslinking chemistry. The invention further relates to a method of patterning a substrate employing the negative resist composition.
2. Related Art
Photolithography is a process of transferring a pattern of geometric shapes on a mask to a substrate such as a silicon wafer. The mask may be a glass plate containing a pattern (e.g., a chromium pattern) of transparent and opaque regions to define the geometrical shapes. Given such a substrate, a layer of photoresist is applied to an exterior surface of the substrate such as by spin coating or the like. There are two types of photoresist: positive photoresist and negative photoresist. Positive resists are insoluble in a developer solution, whereas negative resists are soluble in a developer solution.
For positive resists, the resist is exposed with ultraviolet (UV) light. The UV light is propagated through the mask and onto the substrate, wherever the underlying material is to be removed. In the positive resists, exposure to the UV light changes the chemical structure of the resist so that it becomes soluble in a developer solution. The exposed resist is then selectively washed away by the developer solution, leaving isolated regions of the unexposed resist. The mask, therefore, contains an exact copy of the geometric pattern which is to remain on the substrate.
Negative resists behave in the opposite manner. As is known in the art, exposure to the UV light initiates a cross-linking reaction which causes the negative resist to become polymerized with a consequent significant increase in molecular weight of the reaction product as compared with the molecular weight of the unexposed negative resist. The increase in molecular weight results in the reaction product being insoluble in the developer solution. The cross-linking reaction may be acid catalyzed, and the negative resist may accordingly include an acid generator that generates acid upon exposure to the UV light. Thus, the negative resist remains on the surface of the substrate wherever it is exposed, and the developer solution removes only the unexposed portions. Masks used for negative photoresists, therefore, contain the inverse of the geometric pattern to be transferred.
Traditional negative photoresist compositions characterized by a cross-linking chemistry exhibit disadvantages such as swelling (i.e., expanding in volume) and/or microbridging in photolithographic applications when the exposed photoresist contacts a developer solution or solvent. The swelling and/or microbridging limits the spatial resolution that may be obtained via photolithography. “Microbridging” is said to occur if a continuous strand of photoresist material bridges across a void region in which soluble photoresist has been developed away by a developer solution or solvent, wherein the void region separates two regions of insoluble photoresist material to which the strand of photoresist material is attached.
Therefore, there is a need for negative photoresist compositions that are not subject to swelling and/or microbridging when the exposed photoresist is dissolved in developer solution to avoid limited spatial resolution in photolithographic applications.